The present invention relates to amplifiers and power divider/combiners, and more specifically to spatial power divider/combiners operating in the millimeter wave frequency band.
Recent growth in wireless communication systems has resulted in an increased use of devices operating in the millimeter-wave spectrum. Newly developed wireless communication systems such as Local Multipoint Distribution Service (LMDS) utilize millimeter-wave devices extensively. These systems use medium and high power solid state amplifiers operating in the millimeter-wave spectrum.
While the use of devices operating in this frequency band provides many advantages, one shortcoming of millimeter-wave devices is that they have a very modest power output. Currently, commercially available devices such as monolithic microwave integrated circuits (MMIC) are limited in output power over the millimeter-wave spectrum to approximately one Watt.
In order to achieve higher power outputs, several MMIC devices need to be combined. Power combination has traditionally been achieved using combiner circuits, known as corporate power combiners. FIG. 1a illustrates a corporate power combiner. The power combination is achieved using a series of two-way adders (e.g., Wilkinson combiners). To combine eight inputs, a three stage corporate combiner circuit is used. In the first stage, each of the eight inputs are combined with another input using a series of two-way adders (10a, 10b, 10c, 10d), yielding a total of four outputs (12a, 12b, 12c, 12d). In the second stage, each of these outputs are then paired with another output and a second stage of combination is performed using a second series of two way adders (13a, 13b). Finally, in the third stage, the two outputs (14a,14b) from the second series of two way adders (13a, 13b) are combined in an additional adder 15 to generate a single combined output 16.
While corporate power combiners allow for the combination of several low power MMIC device outputs, such circuits are subject to high loss levels. The efficiency of a corporate power combiner can be represented by:
total efficiency (xcex7)=10xe2x88x92nL/10
where:
L=loss per stage in decibels;
n=number of stages.
The total number of two way adders used in a corporate power combiner circuit increases as the number of stages in the combiner increases, with the total number of adders in a combiner circuit equal to 2n.
The efficiency in a corporate power combiner circuit decreases exponentially as the number of elements combined increases. Because of their low efficiency, the benefits of using of corporate power combiners to combine MMICs are limited.
To improve efficiency, spatial combining techniques have been developed, as illustrated in FIG. 1b. In a spatial combiner, the output of each amplifier 17 is connected to an antenna 18, as shown in FIG. 1b. The antenna transmits the output from the amplifier, and the combining occurs in a spatial electromagnetic field in free space in a single stage. As a result, the efficiency of the combiner is independent of the number of devices being combined. In addition, because a spatial combiner does not contain lossy transmission lines as are used in a corporate combiner, the efficiency of a spatial combiner is significantly higher.
However, despite being more efficient than traditional corporate combiners, spatial combiners still have several drawbacks. Several different spatial combiner techniques have been used to combine signals, but none of these techniques are suited for power combining in the frequency range of 24-36 GHz. Spatial combiners using finline antenna arrays in rectangular waveguides have been used with signals having frequencies up to 10 GHz, but at 28 GHz the cross-section of the waveguide is too small to accommodate finline antenna arrays and devices. Flared coaxial spatial power combiners have been developed which utilize a circular casing loaded with tapered slotline array cards to combine signals with frequencies up to 16 GHz. These devices, however, are very complicated to construct and are space consuming, often ranging up to approximately fourteen inches in length. Spatial combiners using a series of wafers containing patch antennas located between a pair of horn antennas have been used to attempt to combine higher frequency signals. These devices, however, suffer from high rates of power dissipation as the signal traverses the wafer layers. In addition, because of the high rates of power dissipation, it is difficult to provide an effective thermal path in these devices.
Accordingly, there is a need for a power combiner that can be used with high frequency signals (e.g., 28 GHz), has a high level of efficiency, a simple structure, a low profile, an efficient thermal diffusion path, and can be easily and cost effectively manufactured. The present invention fulfills this need among others.
The present invention provides for a spatial power divider/combiner capable of combining high frequency signals. The spatial power divider/combiner in accordance with the present invention provides highly efficient signal combining using a device with a simple structure, low profile, and that can be easily manufactured at a low cost. The present invention provides for both a passive spatial power divider/combiner and an active spatial power divider, or amplifier. A divider/combiner which does not contain any active devices is referred to as a passive divider/combiner, while a divider/combiner whereby the signals are processed (e.g., the signals are amplified) using active devices is referred to as an active divider/combiner, or amplifier.
The spatial power divider/combiner in accordance with the present invention uses a small, simple, rectangular structure. It has a low profile, utilizing a body of less than four inches in length and less than two and one-half inches in width, with a height of less than one inch. The spatial power divider/combiner in accordance with the present invention uses a plurality of slots in an input waveguide to divide the input power into a plurality of equal signals at the same phase. The slots are coupled to microstrip lines to provide a path for the signals to travel to a slotted output waveguide which is identical to the slotted input waveguide. The signals are combined into a single output signal in the slotted output waveguide.
Active devices such as low power solid state MMIC power amplifier devices can be inserted in the path of the microstrip lines to amplify the divided signals prior to recombination.
One aspect of the present invention is a passive spatial power divider/combiner that comprises: a housing containing a first channel forming three sides of a rectangular input waveguide and a second channel forming three sides of a rectangular output waveguide; a board coupled to the housing, wherein the underside of the board forms the fourth side of the input and output waveguides; a series of slots etched on the underside of the board located in the input waveguide to divide an input signal; a series of slots etched on the underside of the board located in the output waveguide to recombine the divided signal; and a series of microstrip lines printed on the top side of the board to couple the input waveguide and the output waveguide.
A second aspect of the present invention is an active spatial power amplifier that comprises: a housing containing a first channel forming three sides of a rectangular input waveguide and a second channel forming three sides of a rectangular output waveguide; a first board coupled to the housing, wherein the underside of the first board forms the fourth side of the input waveguide; a second board coupled to the housing, wherein the underside of the second board forms the fourth side of the output waveguide; a series of slots etched on the underside of the first board located in the input waveguide to divide an input signal; a series of slots etched on the underside of the second board located in the output waveguide to recombine the divided signal; a series of active devices located along the microstrip lines for signal amplification, a series of microstrip lines printed on the top side of the first board to couple the slots in the input waveguide to the active devices, and a series of microstrip lines printed on the top side of the second board to couple the active devices to the slots in the output waveguide.